The word 倏忽 is formed by combining the character 倏, which means "swift" and is structurally composed of the radical 犬 (dog, often associated with quick movement) and the phonetic component 攸, with the character 忽, meaning "suddenly" and composed of the radical 心 (heart, suggesting fleetingness) and the phonetic component 勿. Together, these two characters semantically reinforce each other to create a compound word meaning "in an instant" or "suddenly", through the direct juxtaposition of their similar meanings emphasizing rapid, abrupt change.